Yes, you can use the USB port to output video separate from and addition to the HDMI port on the adapter. For example, I have an iPad acting as an additional display with the iPad's Lightening to USB cable plugged into the USB port on the adapter, while the HDMI port is driving a HDTV. See the iOS APP Store for Duet. You will also need to install the (not on App Store) Duet app on the MacBook. Other USB to HDMI cables/converters/adapters should also work.

As Apple offers this adapter for HDMI output, which is limited resolution, question one is can you then use a HDMI to DVI adapter to connect a DVI monitor? Secondly, the specs page says (in part) "Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display, both at millions of colors" and later "Native DisplayPort 1.2 video output" So that would support this resolution, at unknown refresh rate, but how? Will there be another forthcoming USB-C to mini DisplayPort adapter that could then be re-adapted to Dual-Link DVI or connected to a supported UHD display?

I have a macbook and bought the usb-c digital av multiport adaptor so i could attach an external monitor. I am looking to buy he monitor and am looking at a 4k (UHD) option but not sure about the best or compatible options.

I'm fairly certain it's HDMI 1.4. I tried a 4K screen on the weekend and could not get the refresh rate higher than 30Hz. I was able to drive a Dell 27" at 2560 x 1440 @ 60Hz. Those are the only two screens I've tried so far.